Epidemiological and experimental studies have implicated dietary fat as a possible etiological agent in the development of human breast cancer. Cumulative evidence suggests that lipids may act directly on the mammary epithelium in fostering neoplasia. The glandular epithelium is buried in a matrix of adipocytes whose contents are modified as a function of dietary fat composition. The interactions between the epithelium and the fat cell have been examined in whole animals and in tissue culture systems. It appears that in response to hormonal stimulation the mammary epithelium recruits fatty acids needed for optimal growth (unsaturated fatty acids) from adjacent fat cells. These fatty acids become enriched in mammary epithelial membrane phospholipids and potentiate the hormonal sensitivity of these cells.